1. On Pb-free (solder) Interconnections for High-Temperature Applications A.A. Kodentsov Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology, The Netherlands

2. Cross-sectional view of flip-chip package

3. There is still no obvious (cost-effective) replacement for high-lead, high melting ( 260 - 320  C) solder alloys It is not possible to adjust (to increase above 260  C) liquidus temperature of any existing Sn- based solder alloys by simple alloying with environmentally friendly and inexpensive elements Therefore, in the quest for (cost-effective) replacements of the high-lead solders, attention has to be turned towards different base metals as well as the exploration of alternative joining techniques !

4. Liquidus projection of the Zn-Al-Mg system Ternary eutectic at ~ 343  C

5. The binary Bi – Ag phase diagram

6. TMS 2008 Annual Meeting, New Orleans March 9-13, 2008 “Interfacial behaviour between Bi-Ag Solders and the Ni -substrates” (Hsin-Yi Chuang and Jenn-Ming Song) “Interfacial Reaction and Thermal Fatigue of Zn- 4wt.%Al-1wt.% Cu/Ni Solder Joints” by Y. Takaku, I. Ohnima, Y. Yamada, Y. Yagi, I. Nakagawa, T. Atsumi, K. Ishida

7. The binary Bi – Ag phase diagram

8. The DSC heating curve of the eutectic Bi-Ag alloy

9. Solidification microstructure of the Bi-Ag eutectic alloy (BEI)

10. Solidification microstructure of the Bi-Ag hypo-eutectic alloy (BEI) Ag

11. Transient Liquid Phase (TLP) Bonding solid solid interlayer(s) The interlayers are designed to form a thin or partial layer of a transient liquid phase (TLP) to facilitate bonding via a brazing-like process in which the liquid disappears isothermally In contrast to conventional brazing, the liquid disappears, and a higher melting point phase is formed at the bonding temperature

12. Transient Liquid Phase (TLP) Bonding Any system wherein a liquid phase disappears by diffusion, reaction (amalgamation), volatilization, or other processes is a candidate for TLP bonding ! solid T = const liquid solid solid product T = const Diffusion, Reaction solid

13. The effect of Ni additives in the Cu-substrate on the interfacial reaction with Sn

14. The binary Cu – Sn phase diagram

15. 215  C

16. Diffusion zone morphology developed between Cu and Sn after reaction at 215  C in vacuum for 225 hrs In the  -Cu 6 Sn 5 :

17. Reaction zone developed between Sn and Cu 1at.% Ni alloy after annealing at 215  C for 400 hrs pores !!!

18. Reaction zone developed between Sn and Cu 5at.% Ni alloy after annealing at 215  C for 400 hrs No pores !!! No  -Cu 3 Sn was detected!

19. Isothermal sections through the Sn-Cu-Ni phase diagram P. Oberndorff, 2001C.H. Lin, 2001 235  C240  C

20. Reaction zone developed between Sn and Cu 5at.% Ni alloy after annealing at 215  C for 400 hrs No pores !!! No  -Cu 3 Sn was detected!

21. Diffusion zone morphology developed between Cu and Sn after reaction at 215  C in vacuum for 225 hrs In the  -Cu 6 Sn 5 :

22. 215  C; 1600 hrs; vacuum

23. The binary Cu – Sn phase diagram

24. Part of the Cu-Sn phase diagram in the vicinity of the    / transition Long-Period Superlattice Simple Superlattice

25. 215  C  - phase ?

26. Cu5Ni Sn Cu5Ni (Cu,Ni) 6 Sn 5 250  C Kirkendall plane (s) Cu5Ni Sn Cu5Ni Ag Cu5Ni (Cu,Ni) 6 Sn 5 250  C Cu5Ni

27. Binary phase diagram Ni-Bi 250  C

28. 250  C; 200 hrs; vacuum

30. Parabolic growth of the NiBi 3 intermetallic layers in the binary diffusion couples at 250  C k p = 5.2 x 10 -14 m 2 /s

31. ComponentKnoop hardness (kgf*mm -2 ) Ni113.8 NiBi 3 113.4 NiBi264.8 Cu79.2 Cu 3 Sn464.5 Cu 6 Sn 5 420.8 Knoop microhardness test on Ni-Bi and Cu-Sn systems

32. Cu5Ni Ni Bi Ni NiBi 3 280  C Kirkendall plane (s)

33. 250  C; 400 hrs; vacuum Kirkendall plane(s)

34. Cu5Ni Ni Bi Ni NiBi 3 280  C Kirkendall plane (s) Ni Bi Ni Ag Cu5Ni Ni NiBi 3 280  C Ni

39. Concluding Remarks Through the judicious selection of Sn- or Bi-based interlayer between under bump metallization and substrate pad, (cost-effective) Transient Liquid Phase (TLP) Bonding can be achieved at ~ 250-280  C, and the resulting joints are capable of service at elevated temperatures ! Therefore, in the quest for (cost-effective) substitutes for high-lead solders, attention has to be turned towards different base metals as well as the exploration of alternative joining techniques ! It is not possible to adjust (to increase above 260  C) liquidus temperature of any existing Sn-based solder alloys by simple alloying with environmentally friendly and inexpensive elements The TLP Bonding should be taken into further consideration as substitute for the high-lead soldering !